1. Field of the Invention
This invention generally relates to methods and systems for wafer edge detection and inspection.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail.
Information beyond simple defect detection is often not generated during inspection processes. For example, defect characteristics such as size, magnitude, and location may be determined based on information generated by wafer inspection. However, such information is typically not sufficient to determine defect classifications. Therefore, after wafer inspection, additional information for the defects detected by inspection may be generated using a defect review tool and defect classification is then determined based on the additional information. In some such instances, defects found by an optical defect finding apparatus may be reviewed using a high resolution scanning electron microscope (SEM) review tool.
In order for the defect review to be successful, it is necessary to know the locations of the defects detected by inspection with relatively high accuracy with respect to some fixed location on the wafer. For example, during wafer inspection, the defect coordinates may be determined with respect to a fixed location on a wafer. Therefore, once the wafer is transferred from the wafer inspection system to the defect review tool, the defects can be found by the defect review tool based on the coordinates reported by the wafer inspection system and the fixed location identified by the defect review tool. The fixed location on the wafer can be a center of the wafer and/or a notch formed in the edge of the wafer. As such, determining the coordinates of such fixed locations with substantially high precision during inspection can substantially reduce the difficulty of finding the defects during defect review.
Some methods used by wafer inspection systems for detecting an edge and a notch of a wafer are based on multiple scans and are compatible with wafer inspection tool architectures that scan a spot in a spiral manner over a wafer. In those architectures, wafers may spin up to 100 rotations per second. The wafer inspection systems may utilize an illumination spot with a size ranging from a few microns to tens of microns. Optical collection subsystems of such systems may collect light scattered by the wafer surface and detect the presence of defects of interest (DOI) on the wafer based on changes in the scattering signal. In order to achieve minimum inspection time, the spot path on a wafer is a spiral track with the pitch of a spiral defined by spot size.
The relatively small pitch of a spiral track allows the edge detection system to collect data over multiple revolutions without impacting tool throughput. The same considerations make a single relatively high speed photosensitive element a natural choice for currently used edge detection methods.
Alternative approaches used in some other wafer inspection and metrology tools rely on imaging sensors used to take a limited number of pictures or images of the wafer edge, e.g., 3 to 4. Edge coordinates are found in each of those images and are then used to calculate center of wafer coordinates. Notch detection may require either a whole wafer edge scan or preliminary information about notch position (e.g., from a pre-aligner). Such approaches are typically not integrated with the spiral scanning wafer inspection systems.
Edge detection systems that have edge inspection capability may provide users with additional value. However, currently used single-detector systems rely on gradual changes in detector signal over multiple tracks and therefore may have limited resolution, especially when compared to direct imaging systems, which limits their edge inspection capabilities. For example, a defect may manifest itself as a relatively small change in the signal of a single detector and therefore it would be difficult to detect while relying on gradual track-to-track changes in edge detector signal.
Accordingly, it would be advantageous to develop methods and systems for determining wafer inspection coordinates for one or more fixed locations on a wafer that do not have one or more of the disadvantages described above.